Method of increasing emissivity of tantalum



METHOD OF INCREASING EMISSIVITY OF TANTALUM Robert J. Dwyer, Pasadena, Calif., assignor, by mesne assignments, to Lane-Wells Company, Los Angeles, Calif.,

a corporation of California No Drawing. Application March 2, 1953, Serial No. 339,913

3 Claims. (Cl.204-56) This invention relates toelectrochemical treatment of metal surfaces and particularly to treatment of a metal surface to increase its heat emissivepower.

This invention finds its most important application in the treatment of the surface of tantalum electrodes, particularly the tantalum anodes of the type employed in electron tubes, to increase their heatradiation ability whereby such electrodes will either run cooler for a given power dissipation or for the same temperature conditions have increased power dissipation capacity.

This application is a continuation-in-part of copending application Serial No. 53,381, filed October 7, 1948, now abandoned.

In electron tubes of thetype and construction where radiation of the heat from a tube element is relied upon,

for its cooling, the power dissipation at such an element is limited mainly by the maximum permissible temperature of the material from which the element is constructed. For example, in a rectifier tube having a glass envelope, the cooling of the anodeis dependent upon radiation of the heat through the glass envelope to the surrounding space, air or cooling fluid as theease may be. Thus the maximum permissible power dissipation at the anode is limited to that which can be radiated as heat from the anode when it is operated at its maximum permissible temperature as may be determined, for example, by the softening or melting point of the anode material, by the gassing temperature of the material, or by other temperature limitations of the associated structure of the tube. The maximum permissible temperature of the anode may also be determined by the maximum allowable electron emission therefrom under the particular service to which it is applied. I

Because of its high melting point and other physical properties which suit it to such use, tantalum is frequently used as the material from which the elements of electronic devices areconstructed. For example, in electron tubes of the types employed for rectification, either high vacuum or gaseous, or in amplifier tubes or the like, the anode or so-called plate element is often made of tantalum. This material is usually supplied tothe tube element manufacturer in thin sheet form from which the various tube elements are fabricated. This tantalum sheet as furnished may exhibit either a bright semipolished or lustrous surface finish or a dull unpolished finish. In the first-mentioned condition its reflectivity is relatively high and its absorptivity and emissivity of heat energy are found to be relatively low. In the second-mentioned condition, the reflectivity is less, and its absorptivity and emissivity may be only slightly greater than that of the brightly polished metal. In other words, the tantalum sheet as thus supplied, and as it usually appears in the finished product whether of a dull or polished appearance visually, is a relatively poor heat radiator as compared to that of a black body. Various ways of improving the heat radiating ability of such metals, and which would be applicable, effective, and durable under the special and rigorous conditions preva- 2,720,488 Patented Oct 11,

lent within. the types of electronic devices hereinbefore only slightly beneficial effects. With this and other,

similar processes, the total heat emissivity is apparently not necessarily altered materially, the: surface. merely being changed from a polished or semi-polished reflector to a diffuse reflector. Other methods of modifying the metal surface character to increase its emissivity which are well known to be effective under ordinary conditions and which have heretofore been employed or contentplated, such as the application of a suitable dark coloring matter or coating material, are obviously prohibited by the conditions which must be maintained within a vacuum tube.

It is, therefore, an object of this invention to provide a method for modifying the surface characteristics of tantalum to increase its effective heat emissivity.

It is another object of this invention to provide a method for treating the surface of tantalum toincrease its heat emissivity which will be practicable of use and durable under the conditions heretofore outlined in con-v nection with electron tube devices.

It is another objectof this invention to increase the heat radiation power or thermal emissivity of a tantalum.

electron tube element.

It is a further object of this invention to provide an electron tube element material having an improved emissivity.

In carrying out the process of the present invention,

the tantalum sheet material or the tantalum element to treat is immersed in an electrolytic solution consisting of a solution of sodium hydroxide, preferably C. P.

sodium hydroxide in distilled water. The negative terminal of the source of direct current is connected to a suitable electrode element, preferably one which will be substantially inert with regard to the electrolytic solution, and of sufiicient size to make a relatively low re sistance contact with the electrolytic'solution, and this electrode element is immersed in the solution as the cathode to complete the circuit. Molybdenum has been. found to be a satisfactory material for this negative elec trode. sistance, may be placed in the circuit in series with the direct current source and the electrode and adjustedto provide for a suitably controlled flow of current.

It has been found that the concentration of the sodium hydroxide solution which may be used in this treating process, may be any one falling within the range of from approximately nine per cent to saturation or, in other Words, within the range of from approximately 2% normal to saturation. The preferred solution concentrations for this purpose have been found to fall between.

approximately 3 normal (10%) and approximately 8 normal (24% with a concentration of approximately 7 normal appearing to produce the best results.

When the current source is connected through the circuit across the electrodes immersed in the electrolyte, as hereinbefore described, and the potential thus applied between the electrodes is gradually and slowly increased from a very low voltage, the resultant treatment passes through two principal stages:

Suitable current adjusting means, such as a re-,.

In. the first stage, immediately after the current source isinitially connected to the treating, circuit and. a low voltage is applied across the electrodes, the entire immersed surface area of the tantalum will be observed to commence to change color. The coloration may start at an applied p'ot'entiai'difierence of, for example, approximately l volts, as a brown color, become blue-violet at volts, and a light blue or lavender shade at approximately volts. If the immersed tantalum surface initially carries a lustrous or polished surface finish, as it usually does, the thus colored surface will also retain an equally well polished and lustrous appearance. The coloring of the surface is believed to be due to interference between the light waves reflected from the inner and outer surfaces of a transparent, highly resistant, dielectric coating formed on the tantalum surface by the electrolytic action. A leakage current flows throughv the thus formed dielectric coating and through the electrolyte between the electrodes, and this leakage current increases with such increase in potential until it reaches a value in the order of 0.1 to 0.2

milliampere per square centimeter of immersed tantalum surface just prior to reaching the breakdown potential of the coating, which occurs somewhere between approximately 30 and approximately 50 volts, as hereinafter described in connection with the second stage of treatment.

In the second stage of treatment, when the current source or circuit is adjusted so as to tend to further increase the voltage applied across the electrodes to a value higher than that employed in forming the initial colored coating in the first stage of the treatment, then at some potential difference between approximately 30 and approximately 50 volts, the colored surface coating appears suddenly to commence to break down, as is evidenced by the appearance, at some point on the immersed, blued area of the tantalum electrode, of a bright blue point of light having the appearance of a substantially continuous spark or row of sparks, having an apparent size or row length of from a fraction of a millimeter to one or two millimeters, depending upon the current density of the spark or sparks. At this point in the treatment, coincident with the occurrence and appearance of the aforesaid point of light or spark or row of sparks, the current abruptly increases over and above the before-mentioned leakage current by an amount of at least approximately 0.3 ampere, depending upon the circuit characteristics andconditions, with. the potential between the electrodes, however, remaining within the before-mentioned range of from approximately 30 to approximately 50 volts. Attempts to further increase the voltage results in decreased resistance through the spark with resultant increase in breaking down. or altering the initially formed, blued sur-.

face coating, as it creeps to and fro over the electrode surface, and leaving behind in its path and in the areas .which it has traversed a surface having a uniformly dull,

fiat or etched texture and a color which. is grayish with a faint lavender cast. This second stage of the process con tinues for several minutes until the entire initially blued, immersed area of the tantalum electrode has been trav ersed by the creeping point or row of sparks and all of the initially formed, colored. surface coating is thereby transformed. to a dull lavender-gray surface, as before described... At this stage in the process the creeping of the point or row of sparks ceases, and the electrolytic treatment is then complete. This second and final coating. thus formed in. the second stage of treatment has a susbtantially higher dielectric strength than the colored coating initially formed in the first stage of treatment.

The current during this latter stage of the treating process, except for the before-mentioned relatively lower leakage current portion thereof, which is dependent upon the immersed. area, results substantially entirely from the conductivity of the before-described spark or sparks and is substantially independent of the electrode area. This current in the moving spark or sparks, and exclusive of the leakage current, is referred to hereinafter and in the claims as the spark current. A subsequent increase in the spark current above and outside of the before-mentioned current range produced by a decrease in the series resistance or other suitable adjustment of the current supply in an attempt to increase the voltage, merely results in greater size and rapidity of movement of the creeping point, spark or row of sparks, without substantial change in voltage between the electrodes. 'In other Words, after the initial, blued coating has commenced to be broken down or altered by the creeping point or row of sparks, as before described, the spark current may be varied from not less than approximately 0.3 amperes to several amperes without substantial deviation outside of the before-mentioned electrode potential range of from approximately 30 to approximately volts. Av preferred spark current under the foregoing conditions has been found to be approximately one ampere.

Following the completion. of this second stage of treatment, the tantalum. element is disconnected from the current supply circuit, removed from the electrolyte, and washed or preferably boiled in distilled water, followed by drying. The thus treated surface is found to have had its thermal emissivity or heat radiating ability greatly increased over that of the untreated material and to retain this property without deterioration under ordinary conditions of use within an electron tube.

Instead of employing two stages of treatment, as before described, the whole treating process may be performed in What is, in effect, substantially a single stage of treatment, A typical example of a single stage treating process with suitable circuit characteristics and current and voltage values is as follows:

A tantalum sheet having a total surface area of approximately one and one-quarter (1%) square inches is immersed in a 7 normal aqueous solution of sodium hydroxide contained in a suitable vessel, preferably a jar or beaker of glass or other suitable insulating material. A molybdenum electrode of similar area is also immersed in the solution adjacent the tantalum sheet. The positive and negative poles of a direct current source having a potential of 1.35 volts are connected to the tantalum sheet and the molybdenum electrode, respectively, through a series resistance of approximately 100 ohms. The solu tion is maintained at approximately room temperature. Under these conditions, the preliminary, colored coating is formed on the immersed electrode surface in approximately three seconds, followed almost immediately by the formation and maintenance of the roving or creeping spark action, as before described. During this creeping spark action, a flow of approximately one ampere at a voltage drop of approximately 35 volts is maintained between the tantalum anode and molybdenum cathode in the electrolytic solution. After a treating interval of approximately three minutes, all of the hereinbefore described steps of the process are complete, the creeping spark having crept, moved, or roved during this time over the entire immersed surface area of the tantalum sheet and having by this action transformed the said immersed surface area from its initial, lustrous, or silvery appearance, to a uniform, dull, lavender or violet-gray texture.

The results of the hereinbefore-described processes are substantially independent of the temperature of the electrolytic solution in the neighborhood of room temperature, and any temperature falling between 0 and C. has been found to be satisfactory. Increase in temperature appears in general to result in correspodingly increased rapidity of. the treating action.

The areaof molybdenum cathode used in the beforer described treating processes and the spacing between the molybdenum cathode and tantalum sheet anode are such that with the treating currents employed, a negligible proportion of the voltage drop therebetween occurs through the electrolytic solution and between the cathode and the solution. In other words, under all such treating conditions the major potential drop occurs through the coatings formed on the immersed tantalum surface, and hence the potential difference between the tantalum sheet and the molybdenum electrode is for all practical purposes substantially the same as the potential difference between the tantalum sheet and the electrolyte in which it is immersed. The currents and voltages referred to herein can therefore be considered as existing between the tantalum anode and the solution.

The process of this invention results in the greatly improved thermal emissivity of the tantalum by reason, it is believed, of some chemical reaction or coating formation on or by some mechanical or physical alteration of the surface of the tantalum brought about by the sparking action herein described. The improved emissivity may be the combined result of both chemical and physical effects of the before-described treating process, although the mere physical roughening or dulling of the surface polish of the metal alone has proved to be relatively ineffective for this purpose.

While the term spark has been employed herein in connection with the description of the treating phenomenon of this invention, this is not intended necessarily to be limiting since such spark appears to have at least some of the characteristics of an arc, such as for example, the property of decreasing resistance with increasing current tending to maintain the voltage thereacross constant.

It is to be understood that the foregoing is illustrative only, and that the invention is not limited thereby but includes all modifications thereof Within the scope of definition of the appended claims.

What is claimed is:

1. A method of increasing the thermal emissivity of a surface of tantalum comprising: immersing and subjecting the said surface of tantalum to treatment as the anode in an aqueous solution of sodium hydroxide having a concentration of at least approximately 2 /2 normal, said treatment comprising applying a potential difference between the tantalum surface and said solution for a time sufiicient to form an initial, colored, high resistance coating on said surface; thereupon increasing the current until a substantially continuous, creeping electric spark appears on said initially coated surface immersed in said solution, the current flow through said spark being substantially independent of the area of said immersed surface of tantalum, and said spark maintaining a substantially constant voltage thereacross with increasing current therethrough; and maintaining said spark until the spark has crept over that portion of the area of the intmersed portion of the tantalum which is to have its thermal emissivity increased and such portion of the surface has changed to a dull grayish t0 grayish lavender color.

2. A method of increasing the thermal emissivity of a surface of tantalum comprising: immersing and subjecting the said surface of tantalum to treatment as the anode in an aqueous solution of sodium hydroxide having a concentration of between approximately 3 normal and 8 normal, said treatment comprising applying a potential. dilference between the tantalum surface and said solution for a time sufficient to form an initial, colored, highly resistant coating on said surface; increasing the current until a substantially continuous, roving electric spark appears on said initially coated surface immersed in said solution, the current flow through said spark being substantially independent of the area of said immersed surface of tantalum, and said spark maintaining a substantially constant voltage of between about 30 and about 50 volts thereacross with increasing current therethrough; and main-, taining said spark until the spark has crept over that portion of the area of the immersed portion of the tantalum which is to have its thermal emissivity increased and such portion of the surface has assumed an etched appearance of dull grayish to grayish lavender color.

3. A method of increasing the thermal emissivity of a surface of tantalum comprising: immersing and subjecting the said surface of tantalum to treatment as the anode in an aqueous solution of sodium hydroxide having a concentration of at least approximately 2 /2 normal; applying a potential difference between the tantalum surface and said solution of a value to cause a substantially continuous, roving electric spark on said surface immersed in said solution, the current flow through said spark being substantially independent of the area of said immersed surface of tantalum, and said spark maintaining a substantially constant voltage of between about 30 and about 50 volts thereacross with increasing current therethrough; and maintaining said spark until the spark has roved over the entire portion of the area of the immersed portion of the tantalum which is to have its thermal emissivity increased and such portion of the surface has assumed a dull grayish to grayish lavender color.

References Cited in the file of this patent UNITED STATES PATENTS 2,538,053 Smith Jan. 16, 1951 FOREIGN PATENTS 362,699 Great Britain Dec. 10, 1931 OTHER REFERENCES Annalen der Physik (1907), vol. 328 of complete series (vol. 23 of 4th series), pages 230-239 of article by Schulze. 

1. A METHOD OF INCREASING THE THERMAL EMISSIVITY OF A SURFACE OF TANTALUM COMPRISING: IMMERSING AND SUBJECTING THE SAID SURFACE OF TANTALUM TO TREATMENT AS THE ANODE IN AN AQUEOUS SOLUTION OF SODIUM HYDROXIDE HAVING A CONCENTRATION OF AT LEAST APPROXIMATELY 2 1/2 NORMAL, SAID TREATMENT COMPRISING APPLYING A POTENTIAL DIFFERENCE BETWEEN THE TANTALUM SURFACE AND SAID SOLUTION FOR A TIME SUFFICIENT TO FORM AN INITIAL, COLORED, HIGH RESISTANCE COATING ON SAID SURFACE; THEREUPON INCREASING THE CURRENT UNTIL A SUBSTANTIALLY CONTINUOUS, CREEPING ELECTRIC SPARK APPEARS ON SAID INITIALLY COATED SURFACE IMMERSED IN SAID SOLUTION, THE CURRENT FLOW THROUGH SAID SPARK BEING SUBSTANTIALLY INDEPENDENT OF THE AREA OF SAID IMMERSED SURFACE OF TANTALUM, AND SAID SPARK MAINTAINING A SUBSTANTIALLY CONSTANT VOLTAGE THEREACROSS WITH INCREASING CURRENT THERETHROUGH; AND MAINTAINING SAID SPARK UNTIL THE SPARK HAS CREPT OVER THE PORTION OF THE AREA OF THE IMMERSED PORTION OF THE TANTALUM WHICH IS TO HAVE ITS THERMAL EMISSIVITY INCREASED AND SUCH PORTION OF THE SURFACE HAS CHANGED TO A DULL GRAYISH TO GRAYISH LAVENDER COLOR. 